Fluorescence probe techniques are being used to study the role of membrane fluidity or disorder in the mechanism of penetration of colicin El into the cell envelope. Arrhenius plots of the rate of colicin action are biphasic and show a break in temperature in the range where there is also a decrease in fluorescence polarization. It is proposed to further study these problems with fatty acid auxotrophs and with a fluorescence probe bound covalently to the colicin. The rate of change of phospholipase activity and fluorescence probe response seems to indicate that phospholipase A activity is not principally associated with the decrease in intracellular ATP and potassium levels caused by colicin El.